Direct chill casting is a technique in which aluminum or other molten metal is poured into the inlet end of an open-ended mould while liquid coolant is applied to the inner periphery of the mould to cool the mould plate and generate primary cooling. Also, the same or a different coolant is normally applied as secondary cooling to the surface of the ingot as it emerges from the outlet end of the mould, to continue the cooling effect on the solidifying metal. Where possible, the coolant is applied around the periphery of the mould or a portion thereof, as well as to the faces of the emerging ingot, to make the cooling effect as uniform as possible. However, because of the cross-sectional nature of the mould, the ingot does not cool at a uniform rate throughout the entire cross-section thereof and, moreover, the rate tends to vary not only with the location of the solidification profile in the ingot, but also with the rate at which the metal is being poured into the mould, the nature of the alloy being cast, the metal temperature and the casting speed. The metal along the side walls of the ingot tends to cool and shrink at an uneven rate, with the result that the side walls tend to withdraw inwardly a maximum amount at their centers and lose their flatness.
To obtain flat ingots, moulds have been devised which are capable of forming a crown on the wider side walls of a rectangular ingot to compensate for the uneven shrinkage which these side walls experience as the ingot solidifies. Also, moulds have been devised which are capable of adjusting the degree of deflection in the crown formed on these side walls of the ingot when the casting speed of the mould is increased from the initial low speed during the butt forming stage, to the higher operating speed during the remainder of the operation. For instance, U.S. Pat. No. 4,030,536 describes a system in which the relatively longer sides of the mould are flexed during the moulding operation to adjust the crown imparted to the wider side walls of the ingot.
While moulds of this type can provide a variable crown on the wider side walls of the ingot, there remains a problem of uneven cooling of the ingot because of an irregular impingement point of the coolant on the ingot. Thus, the ingot shrinks as soon as solidification begins so that the impingement point in standard moulds is in effect variable. This means that heat extraction is also non-uniform, especially in the center of the ingot where the shrinkage is highest.
Canadian Pat. No. 1,188,480 describes a direct chill casting method in which the impact point of liquid coolant on the emerging ingot can be varied nearer and farther away from the discharge end of the mould. This is done by directing a first coolant stream at a shallow angle in the direction of metal movement and providing a second coolant stream which converges with the first coolant stream such that by varying the volume and/or velocity of one or more streams, the point of coolant impact on the emerging ingot can be controlled.
It is an object of the present invention to provide a means for adjusting the coolant flow direction dependent upon local shrinkage conditions so that uniform impingement points and preferably constant relative impingement angles can be maintained over each face of the emerging ingot.